In many continuous chemical processes, the continuous monitoring of pH in a stream of flowing fluid is necessary to ensure process control and product quality. Flow-through pH monitoring stations are commonly used for this purpose.
In some applications, however, the placement of a pH electrode in the process stream is undesirable due to the risk of breakage of the electrode and resulting contamination of the process stream. In the food process industry, for example, all possibility of product contamination must be avoided. Glass electrodes, normally the most useful type of process pH electrode, can thus be used only if placed in a side stream which is bled off the process stream and then directed to waste once it has passed the electrode.
Current devices for on-stream pH monitoring require flow rates of 0.25 gallons per minute (1.0 liters per minute) or higher in order to provide an accurate reading. At lower flow rates a lag time exists between the sample point and the electrode signal and the flow passages and electrodes quickly become fouled, particularly when the process fluid is a suspension. High flow rate bleeds are undesirable where the wasted portion represents a substantial economic loss. This is particularly true in the food processing industry where the wasted portion of the process stream cannot be recovered for sale.
There is therefore a need for a flow-through pH-measuring device designed for installment in a bleed stream off the process line to be analyzed, capable of providing a reading substantially current with respect to the process line, yet permitting a low volumetric flow rate with minimum opportunity for fouling.